How to improve soil fertility

Your production of nutritious homegrown grass and forage depends on healthy and fertile soil. Not only can fertile soil bring you economic gains – but it also delivers environmental benefits, giving you the best of both worlds. This guide will help you improve soil fertility by covering key aspects, including organic matter, pH, soil types, and more.

What is soil fertility?

Soil fertility is the ability of soil to supply nutrients and sustain plant growth. It is the combined effect of three major interacting components: the chemical, physical, and biological characteristics of soil.

1. Biological

This encompasses the living organisms in the soil, including bacteria, fungi, nematodes, earthworms, and more. These organisms break down litter and cycle nutrients through the soil. For instance, earthworms help create a healthy soil structure by eating decomposing plant material and creating channels through which air and water can permeate.

2. Physical

Referring to the physical structure of soil, this includes particle size and compaction, ability to store water and allow drainage. When soil is compacted, water struggles to flow through, potentially leading to waterlogging. Root growth and air movement can be stunted by soil compaction.

Good physical structure will enable rain infiltration, which reduces runoff and soil erosion while encouraging soil aeration. A good structure will also benefit the root development of plants.

How to improve soil fertility

3. Chemical

Chemical covers the pH and nutrients in the soil. Organic matter is a store of nutrients including nitrogen (N) and phosphorous (P), which are available to the soil through mineralisation. Each 1% of soil organic matter represents approximately 1,000kg of organic nitrogen and other nutrients.

This can be useful in explaining why grassland farms that are converted from long-term cultivation often yield less than expected in the early years. While arable farms are generally acceptable in terms of P and K content, they are often low in soil organic matter.

The impact of soil pH on nutrients

Fertile soil will contain major nutrients, secondary nutrients, and trace elements to support plant nutrition.

How to improve soil fertility

AHDB data has found that 57% of grassland soil samples are suboptimal in pH and vital nutrients, preventing grass and forage from performing their best.

Nitrogen exists in different forms and can be introduced to soil via nitrogen fixing legumes, artificial fertiliser, animal manure, or the breaking down of crop residue and soil organic matter.

Soil analysis should be your first step to maximising production. With clear analysis, you can act efficiently and cost-effectively to improve soil fertility and support grass and forage production.

Checklist: How to improve soil fertility

  1. Plan your fertiliser needs: Fertiliser is a costly input, so apply it efficiently based on field requirements – a GPS system can help. Fertiliser should only be applied in suitable weather conditions – minimising fertiliser runoff will protect watercourses.
  2. Understand your soil type: Work to enhance and protect soil structure.
  3. Do a soil test: Know the chemical makeup of your soil and understand the nutrients needed to drive forage growth. Measure and monitor for best results.
  4. Lime: Apply if a requirement is identified in your soil test. Adjust soil pH based on what you plan to grow. The optimum is 6.3-6.5 for grass and 6.5-7.0 for grass-clover leys.
  5. Maintain P and K levels using soil index: This will depend on your stocking rate and production system. Check if you have a P allowance and aim to correct deficits where possible. Slurry is an important part of maintaining P and K levels. Aim to meet your P and K requirements rather than exceed them.
  6. Test slurry: This is to quantify the value of your slurry. Slurry analysis is the best method to ensure you are not over- or under-fertilising a crop.

Managing soil types

Peaty soils

On peaty soils, apply phosphorous (P) and potassium (K) where required in the growing season because peat is poor at retaining fertiliser.

Sandy soils

Phosphorus should not be applied in late autumn or winter as it is readily leached from sandy soils. The best response to P will be seen in spring and early summer. Applying a little and often is generally best.

K is most effective in small amounts in spring and summer. Where larger quantities are required, apply in autumn.

Sulphur is an essential nutrient

A fundamental nutrient needed for plant establishment, there is a close relationship between nitrogen and sulphur (S) in plant nutrition. Ultimately, soils require S for optimum yield, with AFBI finding that a deficiency can reduce grass yield by up to 30%.

Research from Lancrop Laboratories found that approximately 85% of all soils tested were deficient in S. Therefore, it is important to remember this when managing soil fertility.

The optimum timing for S application is late spring to early/mid-summer. Soils that need sulphur:

  • Sandy and shallow soils – areas with chalk and limestone.
  • Loamy and coarse silty soils – areas with over 200mm rainfall from November to February.
  • Clay, fine silt or peat soils – areas with over 400mm rainfall from November to February.

Recommended sulphur trioxide (SO₃) applications (AHDB, RB209):

  • Grazing – 20-30kg SO₃/ha per 100kg N/ha applied.
  • Silage – 40kg SO₃ before each cut.

How to improve soil fertility image 3-min

High molybdenum soils

Molybdenum (Mo) is an essential trace element but when levels in the soil are high, this can cause copper deficiency in grazing stock. On high Mo soils, avoid increasing pH above 6.

Speak to a vet about copper supplementation to animals and beware of copper toxicity as too little or too much will negatively affect animal health.

Improving soil pH

This is a critical aspect of soil health, with low pH resulting in poor yields and reduced efficiency of applied fertilisers. Achieving the optimum soil pH will:

  • Increase grass production
  • Increase availability of P, K and other nutrients
  • Increase the efficiency of applied fertiliser

Adding lime to soil

Lime is used to correct acidic soil and provide the right conditions for plants to develop. It has a major influence on:

  • Plant development
  • Efficient use of fertilisers
  • Soil structure
  • Bacterial activity in soil

Knowing the relationship between pH and lime means you can successfully manage your soil and grassland. Target the optimum pH of 6.3-6.5 for grassland to get the most from your soil.

Lime is generally applied as ground limestone. The finer fractions <0.15mm will start to correct soil pH immediately, with the remainder working over a 12 to 24-month period.

Lime can also be applied in granular form with 100% of particles less than 0.1mm fineness, meaning it starts to correct soil pH immediately.

How to improve soil fertility

Benefits of lime on soil PH

  • Lime helps nutrient availability and aids soil fertility by correcting ph. Fertilisers and manure cannot be fully effective if soil pH is low.
  • Lime makes a tremendous difference to the productive potential of grassland. Targeting the optimum soil pH will produce profitable and sustainable benefits on-farm.

Soil test

Base your lime application on soil test sample results. Soil sampling is the best way to evaluate pH levels and determine the need for lime.

  • On mineral soils, the optimum soil pH is 6.3 - 6.5.
  • On peaty soils, the optimum soil pH is 5.5.
  • High Molybdenum areas (Mo) pH < 6.2.

When to apply lime

  • After your last silage cut is ideal to lime.
  • At reseeding time: Always use lime when reseeding. It is especially important with minimum tillage to break down the old sward as it decays.

Leave three months between liming and protected urea/slurry to reduce nitrogen loss through volatilisation. Overcome this by applying protected urea/slurry and then apply lime seven to 10 days later.

Applying lime to soil

Plan lime application as part of a fertiliser program. It is a good idea to have the lime on-farm in time for a spreading opportunity.

  • Spreading should be even and accurate.
  • 125kg/ha of granular lime is equal to 370kg of ground limestone.
  • Ideally spread 20% of the farm each year

It is not recommended to exceed 7t/ha (3t/acre) of lime in a single application.

Maintaining P and K using soil index

Soil index 3 is considered optimal for grass and forage production. To maintain soils at index 3, the P and K application should replace the P and K removed – for example, replace offtakes in crops. The rate applied will depend on your stocking rate and production system.

If your soil P or K index is 1 or 2, additional nutrients are needed to build up reserves. But if your soil P or K index is above 3, you can draw on soil reserves and do not need to apply chemical P or K fertiliser.

Slurry testing

Knowing the nutrient content of slurry on-farm will help you:

  • Save money
  • Reduce your chemical fertiliser use
  • Improve soil health
  • Protect the environment

If silage fields are low in P or K, then the silage will also be low in P or K, meaning the slurry will also be lower in P or K nutrients. Variability in the slurry dry matter will affect N, P, and K content of the slurry.

Test slurry with a hydrometer as it can estimate slurry DM content on-farm. This can be a useful tool to estimate the nutrient content of slurry.

Use data to determine supplementary chemical fertiliser needs. By managing the nutrient content of manure and slurry on-farm, you can benefit from cost savings due to reduced inputs of chemical fertilisers. Generally, 3,000 gallons/acre at 7% DM slurry will supply 20kg P and 142kg K.

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